Is 'Numerosity' Humans' Sixth Sense?

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Whether it's determining the number of ships on the horizon or
the number of cookies in a jar, the human brain has a "map" for
perceiving numbers, new research shows.

Topographical maps of the human
brain are known to exist for the primary senses, such as
sight, hearing and touch, but this is the first time such a map
has been found for numerosity, or number sense. The map's layout
allows for the most efficient communication among neurons doing
similar tasks.

Studies in monkeys have shown that certain neurons in the
parietal cortex, located at the back of the brain beneath the
crown of the hair, became active when the animals viewed a
specific number of items. These studies did not find a map for
numerosity, though scientists have long suspected one exists.
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"Scientists have suspected an ordered mapping of numbers for a
long time," said Andreas Nieder, a neurobiologist at the
University of Tübingen in Germany, who was not involved in the
study. "Many laboratories have been investigating this idea
intensively. Finally, Harvey et al. succeeded in convincingly
demonstrating a map of numerical quantity in the human brain," he
added, referring to researcher Ben Harvey, a neuroscientist at
Utrecht University in the Netherlands.

Dots on the mind

In the study, Harvey and his colleagues placed participants in a
magnetic resonance imaging ( MRI )
scanner and showed them patterns of dots that varied in number
over time. They would show one dot over and over, then two dots
over and over, then three dots, and so on.

The researchers used an advanced imaging method known as
high-field fMRI, which allowed them to see fine-scale details of
brain activity. They analyzed the neural responses using
techniques similar to those used to study the parts of the brain
responsible for vision.

The posterior parietal
cortex, responded to the dot patterns in an organized way:
Small numbers of dots were represented in one area, whereas large
numbers were represented in another, the results showed.

The findings, detailed online today (Sept. 5) in the journal
Science, suggest that higher cognitive functions might rely on
the same organization principles as sensory systems do. For
instance, in
face recognition — which is much more complicated than number
sense — objects that look similar might be grouped together in
the brain, Harvey said.

In these topographical maps, a larger brain area was dedicated to
perceiving smaller numbers than to larger ones, in line with
previous findings that number sense becomes less precise as the
quantity of items increases.

Importantly, numerosity is different from mathematical
ability or symbolism. Numerosity only refers to numerical
amount.

People vary somewhat in their ability to distinguish numerosity,
Harvey said. At the extreme, you have savants — individuals, many
of whom have autism or a similar disorder, who possess
extraordinary abilities in math, art or other areas. Some savants
can look at a pile of pick-up sticks, for example, and instantly
know how many there are.